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A green pool is fixable, and usually quicker than you think
You pull back the cover on a warm morning and the water has a green tinge. Or you spot a few dark specks on the steps that weren’t there last week. It’s a sinking feeling, but please don’t panic. Almost every pool owner meets pool algae at some point, and it can nearly always be cleared with a bit of patience and the right routine.
Algae are tiny plant-like organisms. Their spores blow in on the wind, arrive on swimwear and toys, and wash in with rain. They’re always around. They only take hold when the water gives them a chance to grow.
In this guide we’ll walk you through how to tell which type you’ve got, why it turned up, how to get rid of it safely, and how to keep your water clear from then on.
A quick word on safety first
Clearing algae means using pool chemicals, so it’s worth setting some ground rules before you start:
- Always read and follow the product label. It’s there for a reason.
- Wear gloves and eye protection when handling any chemical.
- Never mix chemicals together, even if they seem similar.
- Always add chemicals to water, never water to chemicals.
- Keep children and pets well away while you work.
- Keep everyone out of the pool until the levels are back in the safe range.
Each type of pool algae looks and behaves a little differently
Knowing what you’re dealing with makes the job much easier. There are three main types you’re likely to see in a UK garden pool.
Green algae is the most common and the easiest to clear
Green algae is the one most people recognise. It floats freely in the water, turning it cloudy and anything from pale mint to deep pea green. It can also cling to walls, steps and corners as a slimy film.
The good news is that it responds well to treatment. It grows fast, but it dies fast too once the water is put right.
Black algae is stubborn and roots itself into surfaces
Black algae shows up as small dark blue or black spots, often about the size of a pinhead or a small coin. You’ll usually find it in shady spots, in grout lines or on rough areas of the pool surface.
It’s tricky because it has a tough protective outer layer and tiny roots that grip the surface. Chlorine struggles to get through that layer, so it needs more physical effort to shift. It also tends to come back if even a small patch is left behind.
Mustard algae hides in the shade and can look like sand
Mustard algae is yellowish brown and dusty looking. People often mistake it for sand or pollen sitting on the floor. It likes shady walls and corners and brushes away easily, but it settles back again within hours.
It’s quite resistant to normal chlorine levels and can hitch a ride on swimwear, towels, toys and pool equipment. So cleaning those items is part of beating it.
Is it definitely algae?
A few other things can look similar, so it’s worth a quick check:
- Cloudy water with no colour can come from poor filtration or high pH rather than algae.
- Brown or rusty water can be caused by metals in the water supply.
- Grit that stays put when brushed is more likely to be dirt or sand.
If you rub a spot and it smears or feels slimy, it’s almost certainly algae.
Algae moves in when the water balance slips
Algae don’t appear out of nowhere. Something in the water chemistry or circulation has usually given them an opening. Here are the most common reasons we see:
- Low chlorine. Sanitiser is your main defence. If levels dip, even for a day or two, algae can get going quickly.
- pH that’s out of range. When pH creeps too high, chlorine works far less effectively. You might have plenty in the water but very little of it doing the job. Our guide to lowering high pH and alkalinity explains how to bring it back into line.
- Poor circulation. Still water in corners, behind ladders or around steps is where algae settle first.
- Not running the filter long enough. The filter removes spores and debris before they can multiply.
- Warm weather and sunshine. Heat and bright light speed up growth, and sunlight also burns off chlorine faster.
- Heavy use or a storm. Lots of swimmers, garden debris or heavy rain all add nutrients and dilute your chemicals.
- Skipping the brush. A thin film you can’t see yet will thicken if it’s left alone.
Losing water and topping up often can also throw your balance out, because every refill dilutes the chemicals. If your level keeps dropping, it’s worth reading about finding and fixing a pool leak.
You can clear pool algae in a few steady steps
Whichever type you have, the basic method is the same. You brush it loose, kill it, filter it out and then rebalance. Give yourself a few days and don’t rush it.
The step-by-step method
- Test the water. Check your chlorine and pH before you add anything. A simple test kit or strips will do.
- Correct the pH. Bring the pH into the normal range first so your chlorine can work properly. Many pool owners aim for around 7.2 to 7.6.
- Clean the filter. Backwash or rinse the filter so it’s ready for the extra work ahead.
- Brush everything. Brush the walls, floor, steps and any corners where the water sits still. This breaks up the algae and lets the treatment reach it.
- Shock the pool. Add a shock treatment following the label. It’s usually best done in the evening, as sunlight breaks chlorine down quickly.
- Run the filter continuously. Keep the pump and filter running around the clock until the water clears. This is when the dead algae gets caught.
- Brush again. Brush once or twice a day while the treatment works. It really does make a difference.
- Vacuum the debris. Once the algae has died and settled, it will look grey or white on the floor. Vacuum it out, ideally to waste if your system allows it, so it doesn’t go back through the filter.
- Clean the filter again. It will have trapped a lot of dead algae by now.
- Retest and rebalance. Only let people back in once chlorine and pH are back in the safe range.
If the water is still green after a couple of days, repeat the shock and keep brushing. A badly overgrown pool can take more than one round.
Extra steps for black algae
Black algae needs more elbow grease. Brush the spots firmly and often to break open that protective outer layer. On some surfaces a stiffer brush is used, but check first that it’s suitable for your pool finish, as a harsh brush can scratch some materials.
After brushing, you can apply a suitable algaecide directly to the affected areas, following the label. Then shock the pool and keep brushing daily. Be thorough. If a single spot survives, it can spread again.
Extra steps for mustard algae
With mustard algae, the key is to deal with everything it might be hiding on. Wash swimwear, and clean toys, floats, brushes, nets and the vacuum head while you treat the water. Leave the cleaning kit in the pool during treatment so it gets sanitised too.
A product designed for mustard algae can help alongside a shock treatment. As always, follow the label and never mix products together. Keep brushing and filtering until there’s no dusty yellow layer left.
When to call in a professional
If the algae keeps returning, the water won’t clear, or you simply don’t have the time, it’s absolutely fine to ask for help. A service visit can get to the root of the problem, whether that’s a filter issue, a circulation problem or a chemistry imbalance.
Keeping pool algae away is mostly about routine
Once your water is sparkling again, a few simple habits will keep it that way. None of them take long, and they’re far easier than clearing a green pool.
Keep an eye on your chemistry
- Test your water regularly, more often in warm spells and after heavy use.
- Keep chlorine in the recommended range. For many pools this is around 1 to 3 mg/l (the same as parts per million), but check the guidance for your pool and sanitiser.
- Keep your pH in range so your chlorine stays effective.
- Keep an eye on stabiliser levels if you use stabilised chlorine, as too much can slow chlorine down.
Keep the water moving
- Run your filter every day. In the busy summer months it needs longer than in cooler weather. If you’re worried about electricity, our guide to pool running costs has some useful pointers.
- Clean or backwash the filter as recommended.
- Point return jets so they push water towards corners and steps.
Keep the surfaces clean
- Brush the walls, floor and steps once a week, and more often in summer.
- Skim leaves and debris off the surface before they sink.
- Empty the skimmer baskets regularly.
Use your cover
A good cover keeps out leaves, dirt and a lot of the spores that cause pool algae in the first place. It also cuts down on sunlight reaching the water, which helps your chlorine last longer.
A smoother surface helps
The finish of your pool matters too. Rough surfaces give algae lots of little places to grip. The pools we install are one-piece with a smooth, non-porous finish, so there are no grout lines for black algae to root into. That makes regular brushing quicker and easier. It won’t make algae impossible, as no pool is immune, but it does give you a head start. You can explore our full range of pools to see the finishes for yourself. For more ideas, take a look at our article on saltwater pools.
We also offer servicing and aftercare, so if you’d rather hand over the routine checks, we can help keep your water in good shape all year. You can read more in our guide to robotic pool cleaners.
Pool algae questions we get asked a lot
How long does it take to clear a green pool?
A mild case can clear in a day or two. A pool that has turned dark green may take several days of shocking, brushing and filtering.
Can I swim in a pool with algae?
It’s best not to. Algae can hide bacteria, make surfaces slippery and cloud the water so you can’t see the bottom. Wait until the water is clear and your chemical levels are back in the safe range.
Why has my pool turned cloudy after shocking?
That’s usually dead algae suspended in the water. Keep the filter running, clean it often and vacuum the floor, and the water should clear.
Why does algae keep coming back?
It’s normally down to low chlorine, pH out of range, poor circulation or patches that weren’t fully brushed away. Mustard algae can also come back in on swimwear and equipment that hasn’t been cleaned.
Does algae grow in winter?
Growth slows right down in cold water, but it doesn’t stop entirely. Keep up light maintenance and use a winter cover so you’re not greeted by a green pool in spring.
Should I drain the pool to get rid of algae?
Very rarely. Draining can damage some pools and it’s rarely needed. In almost every case, the right treatment and some patience will clear the water.
We’re here to help with pool algae and anything else
Algae is one of those things every pool owner deals with now and then. The key is to catch it early, treat it calmly and then stick to a simple routine. Test often, keep the water moving, brush regularly and keep your chemistry in balance, and you’ll spend far more time swimming than scrubbing.
If you’d like a hand with water care, servicing or aftercare, or you’re thinking about a new pool with a smooth, easy-care finish, we’d love to hear from you.
Call us on 01483 323 676 or email hello@hilifepools.co.uk for a friendly, no-pressure chat, or get in touch online and we’ll come back to you.
